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4. Conditions for controlled thermonuclear fusion

If we want to obtain controlled thermonuclear fusion with a positive energy balance in the laboratory, we have to heat a deuterium-tritium plasma to very high temperatures (100 million degrees, i.e., more than six times the temperature of the sunís interior) and keep it confined in a limited space long enough for the energy released from the fusion reactions to compensate both for energy losses and for the energy put in to produce the plasma.

In other words, we have to satisfy the conditions expressed by the Lawson criterion, which depend on the plasma temperature.

When the plasma is a deuterium-tritium mixture at 100 million degrees (equal to about 10 keV of energy), with a low impurity content, the Lawson criterion states that the product of the plasma particle density and the confinement time must be higher than 3x1020 m-3 s.

At such high temperatures the problem is how to confine the plasma in a vessel !

Basically, a plasma which is formed of charged particles (deuterium and tritium ions) can be confined by means of a magnetic field. Without a field, the particles would move about at random and hit the walls of the vessel. This would cause the plasma to cool down and fusion reactions would be impossible.

Instead, a magnetic field forces the particles to follow spiral trajectories around the field force lines and hence keep away from the vessel walls.

 

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